Magnets for various motors used in vehicles, wind power generation, and the like are demanded to have still greater magnetic properties in order to meet social needs for downsizing and weight saving of electronic devices, and for energy and resource saving to cope with global warming, which has been becoming obvious. Among various measures taken, development of R2Fe14B-based rare earth sintered magnets having a high magnetic flux density have actively been made.
A R2Fe14B-based rare earth sintered magnet is generally prepared by melting and casting a raw material, pulverizing the resulting raw material alloy for rare earth sintered magnet into magnet alloy powder, molding the powder in a magnetic field, sintering and ageing the molded product. Pulverization of the raw material alloy for rare earth sintered magnets is performed generally by the combination of hydrogen decrepitation effected by subjecting the raw material alloy to hydrogen absorption/desorption and jet milling effected by bombardment of the raw material alloy in a jet stream. The raw material alloy for rare earth sintered magnet contains a R2Fe14B-based compound phase as a main phase (sometimes referred to as the 2-14-1-based main phase), an R-rich phase containing more rare earth metal elements than the 2-14-1-based main phase (sometimes referred to simply as the R-rich phase hereinbelow), and a B-rich phase containing more boron than the 2-14-1-based main phase (sometimes referred to simply as the B-rich phase hereinbelow). It is known that the alloy structure composed of the 2-14-1-based main phase, R-rich phase, and B-rich phase of the raw material alloy for rare earth sintered magnets affects the pulverizability of the raw material alloy and the characteristics of a resulting rare earth sintered magnet.
Patent Publication 1 discloses a rapidly cooling roll for use in production of rare earth alloys. This publication discloses that, by controlling the Sm and Ra values of the cooling roll surface, the rare earth alloy ribbons produced by using the cooling roll are given uniform short axis diameters both in the center and the ends of the ribbons.
Patent Publication 2 discloses a method of producing rare earth-containing alloy ribbons. This publication discloses that chill crystals and regions with extremely finely dispersed R-rich phases may be reduced by the use of a cooling roll which is provided on its surface with generally linear irregularities extending at an angle of not less than 30° with respect to the rotational direction of the roll to have a particular Rz value.
Patent Publication 1: JP-2002-59245-A
Patent Publication 2: JP-2004-181531-A